In recent years, an advance in fine processing technology has made it possible to impart a nanoscale fine relief structure on the surface of a molded article. The industrial application of the nanoscale fine relief structure has been actively attempted since the nanoscale fine relief structure exerts the function derived from its structure such as an antireflection function called the moth-eye effect and a water repellent function called the lotus effect.
There are a variety of technologies to impart the fine relief structure onto the surface of a molded article. Among these, the method to transfer the fine relief structure formed on the surface of a mold onto the surface of the body of molded article is suitable for the industrial production since the fine relief structure can be imparted onto the surface of the molded article by simple and fewer processes. In recent years, a method utilizing an oxide film with a plurality of pores (anodic porous alumina) which is obtained by anodizing an aluminum substrate has attracted attention as the method to simply manufacture a large-area mold having a fine relief structure on the surface. The interval (pitch) between pores increases in proportion to the applied voltage in the oxide film formed by anodization. The method is suitable as the method of manufacturing a mold from the viewpoint that the interval between pores can be relatively easily controlled as well.
In the case of manufacturing a mold utilizing the anodization, a method is suitable in which the anodization is carried out by two separated stages in order to form pores having both a suitable depth and regular arrangement on the mold. In other words, pores suitable for a mold are formed by sequentially performing the following process (1) to process (3).
Process (1): a process of anodizing the surface of an aluminum substrate and regularly arranging the pores regardless of the depth of the pores.
Process (2): a process of removing a part or all of the oxide film formed in process (1).
Process (3): a process of anodizing the aluminum substrate again to form pores having an arbitrary depth while maintaining the regular arrangement after the process (2).
There is a case in which the aluminum substrate has a mirror finished surface by the machining such as cutting or mechanical polishing.
However, the white streak considered to be derived from cutting streaks or polishing streaks, which are formed in machining of the aluminum substrate, appears when the machined aluminum substrate is anodized at a voltage of 40 V or more and thus the surface of the resulting mold is clouded in some cases. The surface of the mold tends to be clouded particularly in the case of anodizing the machined aluminum substrate at a voltage of 60 V or more in order to form an oxide film having a great interval between pores.
The white streak is also transferred onto the surface of the body of molded article in a case in which the fine relief structure is transferred onto the surface of the body of molded article using a clouded mold. The haze of the molded article having the white streak transferred onto the surface is likely to increase and thus the reflectance also increases.
As the method of manufacturing a mold which is hardly clouded even when the machined aluminum substrate is anodized, for example, a method is disclosed in Patent Document 1 in which the aluminum substrate is subjected to the cathodic electrolysis, electrolytic polishing, or etching prior to the first stage anodization process.